1. Field of the Invention
The present invention relates to polyvinylpyrrolidone powder compositions.
2. Description of the Related Art
Polyvinylpyrrolidone is widely used as a safe functional polymer in the form of a powder or an aqueous solution in various fields for applications such as cosmetics, intermediates of agricultural and medical products, food additives, photosensitive electronic materials, and tackifiers, or for various special industrial applications. In particular, a polyvinylpyrrolidone powder is widely used as a viscosity modifier for a raw material solution at the time of producing a hollow fiber membrane, or as a minute hole forming agent for a hollow fiber membrane. For example, Japanese Patent Laid-open Publication Nos. H10-121324, 2002-239348, 2003-245524, and 2006-239576 disclose processes for producing a hollow fiber membrane by extruding a raw material solution containing polysulfone or polyethersulfone as a membrane forming polymer and polyvinylpyrrolidone as a hydrophilic polymer from a spinneret, immersing the extruded raw material solution in a solidifying bath mainly containing water for solidification, and winding up the solidified hollow fiber membrane.
However, the conventional polyvinylpyrrolidone powder contains a great amount of insoluble substances such as gelled substances and impurities. The insoluble substances contained in a great amount in a polyvinylpyrrolidone powder gives rise to, for example, defect products in the production of a hollow fiber membrane or deterioration in filtration performance of the obtained hollow fiber membrane. Currently, for avoiding these problems, insoluble substances are removed by filtrating a raw material solution at the time of producing a hollow fiber membrane; however, if the content of insoluble substances is high, there is a problem that replacement frequency of the filter is made higher to drastically deteriorate the productivity. For example, Japanese Patent Laid-open Publication No. H10-121324 teaches completely removing undissolved particles by filtrating a raw material solution before extrusion from a spinneret (see, in particular, paragraph [0027]).
Moreover, due to its poor thermal stability, if the conventional polyvinylpyrrolidone powder is left in air, the K value is gradually lowered. If the K value differs depending on the storage period from the purchase to the use of a polyvinylpyrrolidone powder, partly because of the instable quality of a raw material, a product having stable filtration performance cannot be obtained at the time of producing a hollow fiber membrane. For this reason, at the time of producing a hollow fiber membrane, the K value is adjusted by carrying out a heat treatment or the like.